In U.S. Pat. No. 4,057,693 issued Nov. 8, 1977 to R. J. Angner et al, there is disclosed a key telephone line circuit which is connected in shunt (parallel) across the tip and ring lead communication path (loop) between the central switching machine and the subscriber's station to control the HOLD, BUSY and IDLE conditions of the station and to provide the necessary visual and audible signaling to the stations.
Shunt control of a line circuit has several advantages, but a principal one is the perfect balance of the tip and ring circuitry at all times. Other advantages include the ease of application of music and tone-on-hold signals and the fact that the circuit can be removed for maintenance in any state but HOLD without disrupting customer service.
The line circuit disclosed in the above-identified patent includes a shunt detector which provides the following functions:
1. To detect CO ringing and yet differentiate between ringing signals and other signals or noise to prevent false ring-up.
2. To detect large voltage transitions generated across the line when the subscriber telset network is connected (OFF-HOOK) across and disconnected (ON-HOOK) from the line.
3. To provide a path for loop current flow through the hold bridge when the circuit is in HOLD and allow detection of CO open intervals.
The above shunt detector is optimized to provide for detection of signals as diverse as ringing and switchhook transients. However, in certain applications, test signals and switchhook flashing (flashhook) signals cause an erroneous output from the above shunt detector. In such applications, during certain operating states large test signals are to be ignored by the shunt detector while under other operating states smaller transient flashhook signals are to be detected by the shunt detector. Thus, existing shunt detectors may erroneously detect rather than ignore the test signals while erroneously ignoring a valid flashhook signal.